In a luminescent material, light of a first spectral distribution is absorbed and is partially converted towards light of a second spectral distribution. Not all absorbed energy is emitted in the form of light. Some energy is converted towards heat because of the Stokes shift of the luminescent material. The Stokes shift is the difference between the positions of the absorption spectral distribution and the emission spectral distribution. Absorbed photons are emitted as photons with a lower amount of energy and the non-emitted energy is transformed into heat.
When the temperature of the luminescent material becomes too high, the operation of the luminescent material is negatively influenced and in certain cases the luminescent material may deteriorate or may be destroyed. When luminescent materials become too warm, their conversion efficiency significantly drops, their light emission spectrum may change, and their life-time is reduced because of deterioration and/or destruction of the luminescent material. Furthermore, if the luminescent material becomes hot, the material in which the luminescent material is dispersed or dissolved, or the material on which the luminescent material is provided, must also be heat resistant and may be subject to deterioration and/or destruction.
Luminescent materials are often applied in a layer and the light which impinges on the layer is often not homogeneously spread over the whole layer. Therefore, in most case, there is a temperature gradient in the layer with luminescent material. Temperature gradients have the additional disadvantage of the generation of stress in the materials which are subject to the temperature gradient.
Published patent application WO2010/002708A1 relates to a light emitting device which comprises a phosphor layer which converts light from a first color towards light of another color. The phosphor layer is provided on top of a recessed housing and the recess comprises a light emitting semiconductor which emits light of the first color to the phosphor layer. Only a portion of the phosphor layer is illuminated with light of the first color and as the result of the Stokes Shift, this portion of the phosphor layer becomes relatively hot. The phosphor layer may also be in contact with a transparent layer and the combination of the phosphor layer and the transparent layer will locally become relatively hot during operation of the light emitting device. In the cited patent application the recessed housing is made of a thermal conductive material which is capable of conducting the heat away from the phosphor layer and/or the transparent layer.
Although the light emitting device of the cited patent application provides means for cooling the phosphor layer, there is, during operating, still a too large temperature gradient present within the phosphor layer and/or the transparent layer with which the phosphor layer is in contact. For example, the transparent layer should decrease temperature difference within the phosphor layer, however, heat transfer from the phosphor layer to the transparent layer is not efficient and the transparent layer itself does not conduct enough heat towards the housing.